1. Field of the Invention
The present invention relates generally to measurement systems, and more particularly, to methods and systems for a plasma anemometer and methods for using same.
2. Related Art
In recent years there has been a renewed interest in obtaining measurements in high-speed, high enthalpy flows where high-bandwidth sensitivity to mass-flux is desirable. The objectives of this research includes clarifying the mechanisms of stability and transition to turbulence caused by high-speed compressible boundary layers, the unsteady aspects of shock-boundary layer interactions at high Mach numbers, and high-speed flows in compressors and turbines where high shear rates and velocity gradients complicate traditional measurement techniques. Improved predictions of transition Reynolds numbers are also needed in aeronautics applications such as high-speed vehicles or earth re-entry systems. Improved information regarding shock interactions are also needed for designing improved supersonic inlets for air breathing engines of supersonic aircraft. Further, improved information regarding the rotating-machinery class of flows is important for the design of a new and efficient generation of turbo fan engines.
Despite the needs for mass-flux measurements in these environments, there is little accurate data with which to develop empirical criteria or to validate numerical simulations that might be used in the design of new equipment. The difficulty largely comes from the extreme experimental regime that includes high aerodynamic (steady and dynamic) loads, high temperatures, small scales, and large gradients in the measured quantities.
Thermal based sensors have traditionally been used for high-bandwidth measurements where high spatial sensitivity is required. These sensors include hot-wires and hot films. The basic principle is that the temperature of the sensors, determined through a resistance-temperature relationship, is a function of the forced-convective heat transfer. The heat transfer is simultaneously a function of velocity, temperature and density. In incompressible flows where density is constant, the sensitivity to temperature can be minimized by operating the sensor at a high overheat temperature, so that the output is principally proportional to velocity. At compressible Mach numbers, this simplification is not accurate, and the effect of all three independent quantities needs to be accounted for in the sensitivity response function of the sensor. This greatly complicates the procedure for calibration of the sensor. Further, small-diameter suspended hot-wires have a lower thermal mass, which improves their frequency response. However, they are very fragile, especially when compared to hot-film probes. Further problems with surface-mounted sensors, such as hot-films, include a possible severe mismatch in thermal expansion coefficients between the sensor and substrate materials that can lead to damaging mechanical stresses in the substrate, even in moderate-enthalpy hypersonic facilities.
In addition to thermally based sensors, plasma anemometers have also considered for measuring flows. For example, prior experimentation into plasma anemometers was conducted by Mettler, Werner, and Vrebalovich. See, Mettler, R., “The Anemometric Application of an Electrical Glow Discharge in Transverse Air Streams, Ph.D thesis, California Institute of Technology, 1949; Werner F. D., “The Corona Anemometer and its Application to Study of the Effect of Stilling Chamber Turbulence on Test Section Turbulence in a Wind Tunnel at Mach Number Three,” Ph.D thesis, University of Minnesota, 1955; and Vrebalovich, T., “The Development of Direct- and Alternating-Current Glow-Discharge Anemometers for the Study of Turbulence Phenomena in Supersonic Flow, Ph.D thesis, California Institute of Technology, 1954, JPL Report 20-81, respectively. These experimental plasma anemometers were be either alternating current (AC) plasma or direct current (DC) plasma anemometers, and demonstrated that a glow discharge may be made sensitive to fluid disturbances. However, these anemometers were limited to frequencies below 700 kHz, which limits frequency response and bandwidth of the sensor. Further, these anemometers were large and cumbersome and not practical for measurements of high speed flows requiring high spatial resolution. Nor were they operated in a constant current mode, which is necessary to maximize sensor response.